How many texts/minutes do you need a month? Most contracts these days are £30 a month or more, unless you look on websites such as e2save, onestopphoneshop, etc. If you prefer buying from a high street store, head to Carphone Warehouse, where they're currently offering 600mins + 500txts on O2 or 500mins + 500txts on Orange (you also get magic numbers and Orange Wednesdays) for £25 a month. However this is done by redemption, so you pay £35 a month, but you'll get £60 back every 6 months to make it £25/month overall.
Do you want a new phone or are you happy sticking with your current one? If you're not too bothered about using your current phone then ring Orange and they should be able to sort some deal out for you.
If you make most of your calls during evenings and weekends, consider a Vodafone contract, as their "Stop the Clock" feature means you can talk for up to 1 hour and only 3 minutes are taken from your monthly allowance.
The best thing to do is to visit your local independent mobile retailer and have a chat with one of the sales consultants, who will be able to find the best solution for you